1. Field of the Invention
This invention relates to a display device utilizing an element which exhibits the so-called electrochromic phenomenon.
Research and development of such electrochromic element have recently been flourishing owing to its versatility in construction, persistence or memorizability thereof from its color forming (or discoloration) to its extinction, availability of various materials for such element, the color tone of which changes in correspondence to variations in applied voltage values, and so forth.
Typical examples of its application are: electronic desk calculators, horological instruments, etc., in which it is used as the element for indicating numerals, characters, and symbols; sign boards; meteorological charts, traffic sign boards, X-ray image display boards, etc., in which it is used as the element for displaying images in general; soft facsimile display bodies capable of being used repeatedly by erasure as a light shutter or facsimile signal recording; and writing boards, capable of being inscribed or erased in the same way as the ordinary blackboards.
2. Description of the Prior Arts
By the term "electrochromic phenomenon" is generally meant a reversible coloring and decoloring phenomenon depending on electric polarity, wherein the coloring (or discoloring) takes place by application of electricity, and the decoloring occurs by application of electricity having a polarity opposite to that of the abovementioned electric current applied at the time of the coloring or color forming, or by heat application, or a combination of both electric and heat applications. In an actual displaying element, however, it is possible to cause the color forming (or electrochromic phenomenon) without depending on the apparent electric polarity.
The mechanism for the electrochromic phenomenon is not always simple. As one of several views heretofore made, the phenomenon is understood to be due to the so-called oxidation-reduction reaction between an electrolyte and a color forming (electrochromic) substance. In this case, the electrolyte and the electrochromic substance are not necessarily different from each other from the standpoint of the constituent material. Therefore, it may sometimes occur that one and the same substance constitutes the electrochromic substance and simultaneously the electrolytic substance. Also, from another standpoint, there is such a view that the phenomenon takes place due to variations in the light absorbing characteristics of the injected electrons which have been injected into the center of the color as is the case with photochromy. In reality, the result of the combination of these various occurrences is understood to be the cause for the electrochromic phenomenon.
Since the electrochromic phenomenon is caused by electrically changing the color which the material possesses inherently, the possible combinations of colors are innumerable. Further, whether the material is capable of transmitting light or not, or whether it is capable of reflecting or scattering the light or not, is not determined by the property of the material per se, but is determined by the method of forming the layer. Consequently, when the material is to be used as a display apparatus, both the light-transmission type and reflection type may be formed. Incidentally, one of the fundamental constructions of the element which exhibits the electrochromic phenomenon is disclosed in U.S. Pat. No. 3,521,941 (Deb et al.). The element as taught in this patent is such a construction that a current carrier permeable insulator is laminated on an electrochromic layer using a transition metal compound such as W0.sub.3, Mo0.sub.3, etc., and the laminated layers are held between a pair of electrodes. For the current carrier permeable insulator as seen in this patented element, there are used various resin films such as polyester, etc., CaF.sub.2, SiO.sub.2, MgF.sub.2, and so on. It is difficult to say that these compounds, however, are satisfactory for practical use in view of the inferiority in the electrochromic efficiency of the element per se, the displayed color density, and response time, in which points there still exists room for further improvement. By the term "electrochromic efficiency" as used herein is meant a proportional constant between the variation in color density and the quantity of injected charge, since the variation in color density in this element is determined by the quantity of the charge injected into this element.